Electric furnaces are commonly used in the metals industries for melting and smelting operations. A typical electric furnace provides a refractory-lined enclosure, capable of withstanding high temperatures in which feed material is melted by one or more cylindrical electrodes that pass axially through circular ports in the roof to maintain an appropriate depth for the melting operation. Once an electrode strikes an arc with the molten bath, it then moves axially up and down within a controlled range to regulate the furnace power input. Raising the electrode leads to higher resistance and therefore less power input and less heat generation, while lowering the electrode leads to less resistance and therefore more power input and more heat generation. During the melting operation the electrode typically moves up and down within a range of about 30 cm relative to the bath level, on the order of about 5000 times per day. The bath level may also move up and down within a range of about 50 cm.
As the electrode power melts the charge material in the furnace, the electrode tips are gradually consumed due to oxidation and erosion. Therefore, to maintain continuous furnace operation, the electrodes are steadily renewed, typically using one of two methods. One way involves “prebaked electrodes” which are pre-manufactured solid carbon sections that can be threaded onto the preexisting column as needed. A second method involves the in situ formation of “Soderberg electrodes” using consumable steel casings which are routinely welded together as needed and filled with carbon paste, which melts and bakes within the furnace. The second method allows for much larger electrodes to be utilized, typically up to 2 meters in diameter where needed.
Significantly oversized ports are typically provided in the furnace roof to allow for the passage and movement of the electrode column in order to accommodate adequate equipment tolerances, combined axial and lateral movements, electrode vibration, irregularly shaped electrodes, as well as to accommodate buildup that commonly accretes to the exterior of the electrode column during operation. This large annular gap around the electrode typically results in the escape of heat, gases, and other furnace material.
A seal assembly is typically provided for each electrode to limit the costly escape of hot gases and furnace dust. As well, a seal provides a thermal and physical barrier between the furnace interior and exterior. Furnace gases frequently contain toxic metallic fumes, as well as SO2 and CO which are extremely toxic, and potentially lethal. Furthermore, inadequate electrode seals can negatively impact furnace efficiency and can result in difficulties maintaining the furnace atmosphere as well as meeting environmental regulations. High temperature gases that are able to flow through the electrode port can also accelerate the wear of the electrode and electrode port, and can lead to dangerous and difficult working conditions.
Maintaining an adequate electrode seal has proved to be an extremely difficult task, and for various reasons. The continuous upward and downward axial movement of the electrode results in the electrode surface constantly scraping against typical electrode seals, posing serious wear challenges in maintaining the seal. The electrode seal is also directly exposed to gas within the furnace freeboard, typically at temperatures of up to about 1500° C., but potentially higher depending on the operation, as well as pressure variations, resulting in large temperature differentials and significant seal material limitations. It is also common for an electrode to move radially off-axis during normal operations, both in position and plumbness, due to high electromagnetic forces, thrusts from charge banks, and due to mechanical tolerances. Soderberg electrodes are particularly prone to shape irregularities due to over or under sized casings, as well as casing deformation often leading to non-circular electrodes. An additional complication arises due to the welding of casing sections. While attempts are made to grind the welded surfaces free of imperfections, it is common for rough welds to damage an electrode seal. Prebaked electrodes are generally more uniform, however, the joint between adjacent electrode sections is frequently off-center resulting in a step at the interface between the two adjoining sections. Material emanating from within the furnace commonly accretes to the exterior of the electrodes as they are inserted and withdrawn through the roof port during normal operation. Soderberg electrodes have an additional risk of carbon paste leaking from within the casing and hardening on the exterior prior to passing through the electrode seal. This and other material buildup often permanently damage an electrode seal as it is repeatedly passed through the seal.